1. Field of the Invention
The present invention relates to an optical pickup device that can be used in information-reproducing apparatus such as compact disc reproducing apparatii, video disc reproducing apparatii and the like.
2. Description of the Prior Art
FIG. 3 shows a conventional optical pickup device that has a semiconductor laser 11, a grating 12, a hologram optical element 13, a collimating lens 14, and an object lens 15. A light beam from the semiconductor laser 11 is diffracted by the grating 12, resulting in three separate beams, one of which is the zero-order diffracted beam (below referred to as the main beam), and the other of which are the firstorder diffracted beams (below referred to as the sub-beams) in the positive and negative directions that are substantially orthogonal to the plane in FIG. 3. These three separate beams are further diffracted by the hologram optical element 13. The resulting zero-order diffracted beam of each of the above-mentioned separate beams enters the object lens 15 via the collimating lens 14 and is focused on a recording medium 16. When the main beam is focused on a pit of the recording medium 16, the intensity of the beam reflected from the recording medium 16 gives a pit signal. The two sub-beams, which are positioned symmetrically with respect to the above-mentioned main beam, are focused on the recording medium 16 in such a way that they shift to a larger extent in the tracking direction of the recording medium 16 and to a small extent in the radial direction of the recording medium 16, thereby generating a tracking error signal from a difference in the intensity between the two sub-beams reflected from the recording medium 16. The beams reflected from the recording medium 16 pass through the object lens 15 and the collimating lens 14 and are diffracted by the hologram optical element 13, and the resulting first-order diffracted beams are introduced into a photodetector 17.
FIG. 4(a) shows the relationship between the configuration of the hologram optical element 13 that is seen from the recording medium 16, and FIG. 4(b) shows the configuration of the photodetector 17. The hologram optical element 13 is divided into two regions 13a and 13b by a division line 13c in the radial direction. The regions 13a and 13b have a number of grating lines that are inclined with respect to the division line 13c and that are symmetrical about the division line 13c. The photodetector 17 is divided into six regions 17a, 17b, 17c, 17d, 17e, and 17f. FIG. 4(c) shows the orientation of the devices of FIGS. 4(a) and 4(b).
When a beam from the semiconductor laser 11 is precisely focused on the recording medium 16 or set at the correct focus, the resulting main beam that has been diffracted by the region 13a of the hologram optical element 13 is focused on the division line A.sub.1 of the photodetector 17 to form a spot Q.sub.1 on the division line A.sub.1. The resulting main beam that has been diffracted by the region 13b of the hologram optical element 13 is focused on the division line B.sub.1, to form a spot Q.sub.2 on the division line B.sub.1. The resulting sub-beams are focused on the regions 17e and 17f of the photodetector 17. when output signals of the photodetecting regions 17a, 17b, 17c, 17d, 17e, and 17f are represented respectively as S.sub.1a, S.sub.1b, S.sub.1c, S.sub.1d, S.sub.1e, and S.sub.1f, a focus error signal is obtained by calculating (S.sub.1a +S.sub.1d)-(S.sub.1b +S.sub.1c), a tracking error signal is obtained by calculating (S.sub.1e -S.sub.1f), and a pit signal (i.e., an information signal) is obtained by calculating (S.sub.1a +S.sub.1b +S.sub.1c +s.sub.1d).
FIG. 5 shows another conventional optical pickup device, which is different from the above-mentioned conventional device in the configurations of both the hologram optical element 23 and the photodetector 27. FIG. 6(a) shows the relationship between the configuration of the grating lines of the hologram optical element 23 that is seen from the recording medium 26 and FIG. 6(b) shows the configuration of the photodetector 27. FIG. 6(c) shows the orientation of the devices in FIGS. 6(a) and 6(b). The hologram optical element 23 is divided into two regions 23a and 23b by a division line 23c in the radial direction. The regions 23a and 23b have a number of grating lines that are at right angles to the division line 23c. The grid pitch of one region 23a is different from that of the other 23b. The photodetector 27 are divided into six regions 27a, 27b, 27c, 27d, 27e, and 27f. When a beam from the semiconductor laser 11 is precisely focused on the recording medium 26 or set at the correct focus, the resulting main beam that has been diffracted by the region 23a is focused on the division line A.sub.2 to form a spot R.sub.1. The resulting main beam that has been diffracted by the region 23b is focused on the division line B.sub.2 to form a spot R.sub.2. The resulting sub-beams are focused on the photodetecting region 27e and 27f. When output signals of the photodetecting regions 27a, 27b, 27c, 27d, 27e, and 27f are represented respectively as S.sub.2a, S.sub.2b, S.sub.2c, S.sub.2d, S.sub.2e, and S.sub.2f, a focus error signal is obtained by calculating (S.sub.2a +S.sub.2d)-(S.sub.2b +S.sub.2c), a tracking error signal is obtained by calculating (S.sub.2e -S.sub.2f), and a pit signal is obtained by calculating (S.sub.2a +S.sub.2b +S.sub.2c +S.sub.2d).
In the conventional optical pickup devices with the above-mentioned structures, the spots Q.sub.1 and Q.sub.2 (R.sub.1 and R.sub.2) based on the beams reflected from the recording medium 16(26) must be precisely formed on the division lines of the photodetector 17(27). To achieve this, a delicate adjustment must be carried out so that the hologram optical element 13(23) and the photodetector 17(27), respectively, can be disposed at a given position. However, in order that the hologram optical element 13(23) and the photodetector 17(27) are constructed to be moved separately, there must be a supporting structure by which the photodetector 17(27) is freely moved. This makes the entire structure of the device complicated, causing difficulties in obtaining a light-weight, miniaturized device. Moreover, a number of positioning parts are needed, which makes the production process of the device complicated and makes the production cost expensive.
Japanese Laid-Open Patent Application 63-13134 discloses an optical pickup device that has the same structure as that of FIGS. 6(a) and 6(b) mentioned above, except that the hologram optical element functions as an anastigmatic lens. The focus error signal of this pickup device is represented by the same calculation equation as that of the focus error signal of the pickup device of FIGS. 6(a) and 6(b). Thus, the pickup device of the above-mentioned Japanese Laid-Open Patent Application has the same problems as those of the pickup device of FIGS. 6(a) and 6(b).
To solve these problems, the present invention incorporates both the semiconductor laser 11(21) and the photodetector 17(27) into the same package so that the positioning of the spots Q.sub.1 and Q.sub.2 (R.sub.1 and R.sub.2) on the division lines of the photodetector 17(27) can be carried out by the positional adjustment of the hologram optical element 13(23) alone. However, in the optical pickup device with such a structure, the slight shifting of the positions of the semiconductor laser 11(21) and the photodetector 17(27) from those of the initial plan makes it impossible to form the beam spots at the correct positions of the photodetector 17(27), resulting in a focus offset. To remove this focus offset, the position of the hologram optical element 13(23) must be adjusted by a forward or backward movement and/or the rotation of the hologram optical element 13(23) so as to shift the spots on the photodetector 17(27) thereby making the focus error signal become zero when the beam from the semiconductor layer 11(21) is at the correct focus on the recording medium 16(26). However, the two spots on the photodetector 17(27) that are formed based on the main beams shift at the same time. In this way, the position of the hologram optical element 13(23) cannot be adjusted without the simultaneous shifting of these beam spots on the photodetector 17(27). Moreover, there is a possibility that the shifting of the two spots are countervailed on the focus error signals corresponding thereto. To avoid this, the hologram optical element must be moved to a great extent in the direction of the y-axis. Especially, with the optical pickup device shown in FIGS. 5 and 6, because the length of each of the divided regions of the photodetector 27 in the direction of the y-axis is short, when a great focus offset occurs and the hologram optical element 23 is moved to a great extent in the direction of the y-axis to compensate the focus offset, the beam spots R.sub.1 and R.sub.2 on the photodetector 27 shift to a great extent in the direction of the y-axis and slip out of the photodetecting regions on which these spots must be formed.
Moreover, because the hologram optical element 13(23) must be moved to compensate the focus offset phenomenon, a photodetector 17(27) that is large enough to receive the beam spots is required, makes the cost of production expensive.